The present invention relates to masking couplers formed from conventional image-producing coupler moieties for color photographic materials. In particular, it relates to masking couplers using azo dyes to supply color to the masking coupler. The masking couplers may be blocked or unblocked.
It is well known that in a color photographic system, a color reproduction method based on the subtractive color process system usually employs cyan, magenta and yellow negative dye images. These dye images are formed when color-forming couplers undergo a coupling reaction with the oxidation product of a color developing agent such as a primary aromatic amine.
The dyes employed in such a system, however, are known to be inefficient with respect to transmitting all of the electromagnetic radiation expected from theoretical considerations. That is to say, a dye which is capable of absorbing radiation in a given region of the spectrum should ideally transmit radiation in all other regions of the spectrum, practical experience shows that such expectations are not realized. For example, a cyan dye is expected to absorb radiation in the red region of the visible spectrum and to transmit radiation in the green and blue regions. In reality, cyan dyes also absorb some radiation in both the green and blue regions of the electromagnetic spectrum. The magenta dye should absorb only green light but, in practice, also absorbs some blue and a small amount of red. The yellow dye is closer to theory, and its unwanted absorptions of green and red light are small. The result of this unwanted side-absorption is distortion in the color reproduction of color photographic materials.
To correct this problem of side-absorption, a masking technique is employed whereby the effect of the undesired side-absorption is canceled out. As described for example in U.S. Pat. No. 2,428,054, and in "The Theory of the Photographic Process", 4th Ed., T. H. James, Macmillan Publishing Co., Inc., New York (1977), pp. 571-574, the conventional masking technique for use with a cyan dye is to employ a colored cyan dye forming coupler, i.e., a masking coupler, which is colored as coated in a photographic element (yellow, and/or magenta) so that it has an absorption in the blue and/or green region. Upon reaction of the masking coupler in exposed areas with oxidized color developing agent, the original yellow and/or magenta color of the masking coupler is eliminated as the cyan dye is formed. The net result of combining the cyan masking coupler and the cyan image coupler is to have the equivalent amounts of blue and/or green density in both the exposed and unexposed areas thus "masking" or effectively removing the effects of the unwanted absorbance of the cyan dyes.
A colored masking coupler should exhibit desirable hue properties such as a narrow bandwidth of absorption and a high level of extinction. In addition, a masking coupler should have good reactivity towards the oxidized color developing agent and solubilization and surfactant properties.
A problem arising from the use of masking couplers which are colored when coated in the film is that the colored masking couplers absorb some of the light which should be absorbed by the silver halide crystals. U.S. Pat. No. 2,860,974 describes a method for alleviating this problem which utilizes a light-colored, non-image-forming colored coupler whose color is then shifted during processing to the desired characteristics for masking, thus avoiding light filtration effects during exposure. The use of color or hue shifting allows non- or low-colored materials to be coated which then become colored in the non-exposed or D.sub.min areas of the film only after processing. Hue-shifting can be accomplished using various known techniques for blocking and unblocking the dye chromophore or auxochrome as described, for example, in U.S. Pat. Nos. 4,690,885; 4,358,525 and 4,554,243, United Kingdom Patent Application No. 2,105,482.
The following colored azo dye structures are typical of conventional masking couplers: ##STR1##
These masking couplers exhibit relatively low extinction levels, broad bandwidths, and limited hue shifting capability. For example, conventional compound A has a low extinction of 31,000 in acetone and undesirable surfactant properties arising from its strong acid (SO.sub.3 H) solubilization. Furthermore, it is very difficult to shift conventional compound A in a practical, easy-to-reproduce manner.
U.S. Pat. No. 4,840,884 discloses a coupler having a coupling-off group which is represented by the formula--L--NR.sup.1 --DYE, wherein L is a particular kind of linking group, NR.sup.1 is a substituted nitrogen atom and DYE can be an azo dye. These couplers, however, generally are not useful as masking couplers because the azoaniline dye is shifted (lambda max&lt;400 nm) due to the carbonyl linking group's direct attachment to the nitrogen atom. As a result, while these couplers are not significantly colored as coated, the residual couplers remain largely uncolored as well after processing and provide little of the desired color correction.